1. Field of the Invention
The invention relates to a motor control device that controls driving of a motor.
2. Description of Related Art
An electric power steering system is known in which a steering operation performed by a driver is assisted by applying a torque generated by a motor to a steering shaft or a rack shaft of a vehicle as an assist torque. The electric power steering system includes a motor control device that controls driving of the motor, based on a steering torque applied to a steering mechanism. The motor control device includes a drive circuit that supplies drive electric power in three phases to the motor, according to turning on or off of switching elements disposed on feeding paths for phases (a U phase, a V phase, a W phase) for the motor, and a microcomputer that generates control signals for turning on or off the switching elements of the drive circuit. The microcomputer computes current command values corresponding to a target output of the motor, based on the steering torque. The microcomputer computes voltage command values of the phases for the motor by performing current feedback control that causes current values of the phases for the motor to follow the current command values, and generates the control signals based on the voltage command values of the phases.
In such a motor control device, when abnormality such as breakage occurs in one of the phase feeding paths for the motor, various types of fail-safe control such as two-phase drive control may be performed. When breakage abnormality occurs, it is necessary to determine which one of the phase feeding paths is a faulty phase, in order to perform the fail-safe control. A conventional motor control device that is able to detect the faulty phase is described in the publication of Japanese Patent No. 4348897. A detection principle is as follows.
When abnormality such as breakage occurs in one of the phase feeding paths, a current value of the faulty phase in which the abnormality occurs is held at zero. Therefore, the current value of the faulty phase is likely to deviate from the current command value. Accordingly, the voltage command value of the faulty phase is likely to remain at an upper limit value or a lower limit value while a motor rotation angle changes. In the motor control device described in Japanese Patent No. 4348897, a focus is placed on this feature, and when one of the three phases is set as a specific phase, it is determined that a current-carrying failure occurs in the specific phase, on the condition that the current value of the specific phase is less than a predetermined current determination value, and the voltage command value of the specific phase exceeds a predetermined voltage command determination value.
When abnormality such as breakage occurs in one of the phase feeding paths, the current values of two phases, which are normal and are other than the faulty phase in which the abnormality occurs, are also changed, and therefore, the current values of the two normal phases are likely to deviate from the current command values. Therefore, the voltage command values of the two normal phases may change in the vicinity of the upper limit value or in the vicinity of the lower limit value while the motor rotation angle changes. Thus, in the motor control device described in Japanese Patent No. 4348897, if the current value of the normal phase is less than a predetermined value when the voltage command value of the normal phase is a value in the vicinity of the upper limit value or in the vicinity of the lower limit value, there is a possibility that the normal phase may be erroneously determined as the faulty phase and this erroneous detection of the faulty phase is undesirable.